1. Field of the Invention
Generally, the present invention relates to subsea wellhead systems, and more particularly to metal-to-metal sealing systems in mudline suspension systems.
2. Description of the Related Art
In many offshore oil and gas drilling operations, subsea wells are drilled using temporary drilling platforms, such as jack-up rigs, floating rigs, and the like, the specific type and design of which may depend on various drilling and environmental factors, such as water depth, target drilling depth, and the like. Unlike those situations where a subsea well is drilled using a conventional fixed offshore drilling platform—which is commonly designed to support a significant portion of the weight of the various casing strings installed in the well during drilling operations—the weight of the well is not borne by the jack-up or floating rig, but is instead typically supported by a so-called mudline suspension system. In a mudline suspension system, the weight of the various casing strings and other well components are supported at the sea bed, or mudline, which thereby enables a substantial reduction in the size and weight of the structural components needed in the structure of the temporary drilling platform, which can primarily be design to provide lateral stability during drilling operations to those portions of the well casing rising above the sea floor.
Typically, once a temporary drilling platform, e.g., a jack-up or floating drilling rig, has completed the drilling and testing operations on given subsea well, the sections of casing that rise above the wellhead at the sea floor are removed, each casing string is capped at the subsea wellhead, and the well may be temporarily abandoned for a period of time prior to the commencement of production. Temporary abandonment of the subsea well may in some cases last for several months, depending on several factors, such as the age of the field, the number of adjacent wells, the presence of and distance to an existing production platform, and the like. Thereafter, once an offshore production platform—which may gather production from a single subsea well or from multiple wells adjacent well—has been brought into position, the well is then reconnected, or tied back, to the production platform, and oil and gas production from the well may begin.
While the use of mudline suspension systems in offshore oil and gas wells may provide substantial benefits in both drilling and production operations, locating the various casing hangers and other related complex connections at the mudline may impose several technical challenges. For example, access to the subsea wellhead may be severely limited, particularly in those cases where the water depth may range to 2000-3000 feet, or even greater. In such cases, routine equipment maintenance and/or repair of the wellhead components may be problematic. Accordingly, great emphasis is generally placed on robust and reliably engineered sealing systems, since any repairs that may be required for leaking or malfunctioning wellheads in a subsea environment are difficult, if not practically impossible, to perform. Sealing system reliability is of even greater importance in high pressure/high temperature (HPHT) wellhead applications, which may operate under pressures in excess of 10,000 psi and temperatures above 250° F.
One particular aspect of prior art mudline suspension systems where sealing system problems and/or failures have occurred is with the various pressure-retaining components such as casing hangers and their related landing, abandonment, and/or tieback connections. As noted above, many subsea oil and gas wells may experience a drilling phase, an abandonment phase, and a production phase, during which time at least some of the seals on a given casing hanger may experience multiple sealing and unsealing cycles, sometimes referred to as “make/break” cycles. The following sealing system life-cycle description is typical for the life of an exemplary casing hanger seal.
Typically, the sealing surface of a casing hanger experiences a first sealing cycle in the shop where it is initially manufactured when it is connected to and tested with a landing subassembly, or landing sub, so called because it is used to “land” or install the casing hanger and casing inside of a drilled wellbore. In most cases, the casing hanger and the landing sub remain in this connected and sealed configuration throughout the step of landing, or installing, the assembled casing, casing hanger and landing sub in the well, after which the casing string is cemented in place. In at least some applications, this first seal between the casing hanger and the landing sub is then broken so that a back-wash step for removing cement and/or other debris from the annular space outside of the casing hanger can be performed. Once the back-wash step is complete, the landing sub is then re-engaged with the casing hanger, thus creating a second sealing cycle on the sealing surface of the casing hanger.
After the completion of all drilling and testing operations on the well, the second seal between the casing hanger and the landing sub is broken so that the casing riser string and the landing sub can be removed in preparation for temporary abandonment of the well. Thereafter, a temporary abandonment cap, or TA cap, is installed on the casing hanger, thus imposing a third sealing cycle on the sealing surface of the casing hanger. The TA cap then remains in place until production operations are ready to commence, at which time the third seal between the casing hanger and the TA cap is broken so that the well can be re-opened and re-connected, or tied back, to a production platform. A casing riser string and tieback subassembly, or tieback sub, that is used for tying the well back to the production platform is then lowered into place and connected to the casing hanger, thus imposing a fourth sealing cycle on the sealing surface of the casing hanger.
As may be appreciated, deformation and/or surface damage to the various sealing surfaces of a typical mudline suspension system can occur during any one or more of the sealing cycles described above. Furthermore, surface damage and/or deformation problems are often much less forgiving in HPHT applications, where metal-to-metal seals are normally employed due to the inherent temperature limitations of many elastomeric seal materials, and the commensurately reduced durability and reliability thereof. Moreover, since high-nickel alloy materials are used for many HPHT wellhead applications, the galling problems associated with sliding engagement of metal-to-metal sealing surfaces are also much more prevalent.
Accordingly, there is a need to develop metal-to-metal sealing systems and configurations that may overcome, or at least mitigate, one or more of the problems associated with the various sealing, unsealing, and resealing operations that are performed on a typical mudline suspension system as described above.